This invention relates to digital communications, and in particular to channelized digital data communications over digital loop carrier wired networks. The invention has particular application in distributed digital switching systems for high-speed digital communications.
The T1 time division multiplexed (TDM) transmission format is a format widely deployed in North America for the transport of multiplexed voice channels. A T1 signal consists of 24 DS-0 channels that are intended to transport an eight-bit voice sample every frame period, with the frame repeating each 125 xcexcs to yield an overall bit rate per DS-0 channel of 64,000 b/s. Each T1 frame includes a single framing bit, resulting in a total of 193 bits per each 125 xcexcs frame for a bit rate of 1.544 Mb/s.
Originally designed to carry voice information over trunk lines, more recent uses of T1 have been data transport and voice and data transport, in which digital data and digitized voice are each allocated one or more DS-0 channels.
Each DS-0 channel has associated with it Channel Associate Signaling (CAS) signaling bits. These may be ABCD bits or AB bits, either of which are used to indicate the channel status of the line with which it is associated, such as telephone off-hook, ring, etc. Since CAS bits do not need to be updated very often, they are typically transported in a T1 format as part of a superframe or an extended superframe constructed by robbing lower significant bits from selected DS-0 channels. Specifically, a superframe is formed from a series of twelve T1 frames by robbing all of the DS-0 voice channels of their least significant bit every sixth T1 frame. Since two robbed bits are used every such superframe, AB CAS transport is allowed.
Similarly, an extended superframe is formed from twenty-four T1 frames by robbing all of the DS-0 voice channels of their least significant bit every sixth T1 frame. Since four robbed bits are used every such extended superframe, ABCD CAS transport is allowed.
If the robbed DS-0 channel is used to carry digital data, then data corruption inevitably occurs, to the detriment of data transmission in that DS-0 channel. In the past, to solve this problem, either the robbed bit signaling has been deactivated for each DS-0 channel or only the seven most significant bits (7 MSBs but not the LSB) are used for data transport. Neither solution has been wholly satisfactory because the former prevents voice transport over the DS-0 channel. It is not always desirable to limit a particular DS-0 channel and the latter significantly reduces the bandwidth available for data transmission. The former also prevents dynamic allocation of voice and data channels.
The TDM communication standard in Europe known as E1 is a similar standard to that of the American T1 standard. However, El transmission format allocates the 17th byte in a 32 DS-0 frame to carry CAS nibbles.
Those of ordinary skill in the art should be aware of the background information found in the following typical publications:
xe2x80x9cFunctional Criteria for DLC Systems,xe2x80x9d Bellcore TR-NWT-000057, Issue 2, January 1993;
xe2x80x9cTransport Systems Generic Requirements(TSGR): Common Requirements,xe2x80x9d Bellcore GR-499-CORE, Issue 1, December 1995; and
xe2x80x9cDigital Interface Between SLC96 Digital Loop Carrier System and Local Digital Switch,xe2x80x9d Bellcore TR-TSY-000008, Issue 2, August 1987.
What is needed is a mechanism to optimize usage of a T1 resource by pooling DS-0 channels to carry data traffic when not used to carry voice traffic.
According to the invention, a universal DS-0 channelized format is provided wherein selected values of CAS bits are used to indicate to the transport mechanism when DS-0 channels are not being used for voice and are available for use for data transport. In addition, the T1 frame format carries CAS bits only in the first DS-0 channel, so that robbed bits are disabled and not used to carry CAS signaling bits, thereby avoiding any risk of data corruption. Still further, a special CAS signaling value not defined as a standard CAS value, is employed in a frame nibble instead of a frame counter to indicate that the state is in the first DS-0 slot.
By allocating an entire DS-0 channel or 1/24th of a T1 frame to signaling bits and datalink bits, the T1 frame actually achieves higher integrity for data, improved data throughput and decreased data loss, thus increasing data capacity without significantly altering the infrastructure underlying T1 signaling.